Measuring Caster And Camber - The Caster Plan

If you aren't trying to race on the Nextel Cup level, you probably aren't using computerized telemetry systems to help gather information and determine the best chassis setups. So working on the chassis is a fairly straightforward affair, essentially requiring only standard tools such as wrenches, ratchet and sockets, measuring tape, and so on. Of course, stock car racers depend on a few specialized tools specifically developed to suit their needs, and without those tools you are at a definite disadvantage at the track.

One of the most important tools in this category is a caster/camber gauge. It is used to measure caster, or the angle the spindle is leaned forward or back, and camber, or the angle the front wheels are leaning in or out. Caster/camber gauges can either be digital or manual, but either way they work the same.

Caster controls how well the car turns. If you are looking at your race car from the side, caster is the angle of an imaginary line between the upper and lower ball joints relative to vertical. If that line is angled so that the top leans toward the back of the car, that's positive caster. If the top of the line leans toward the front of the car, that's negative caster. Positive caster makes the car feel more stable and resists turning. If you are turning a car that has positive caster on both front wheels and you let go of the steering wheel, it will automatically straighten up. Negative caster does the opposite and usually makes the car feel darty. Racers rarely run negative caster.

Tuning with caster is usually accomplished with caster split, or how much more positive caster you have in the right-front than the left-front wheel. The greater the caster split, the more the car will tend to turn left on its own. Typically, you don't need much caster split in a modern race car with power steering: only 2 degrees of positive caster in the left front and 4 in the right front. You don't want too much caster because it will cause tire scrub and burn horsepower.

As already stated, measuring caster is straightforward. Begin by locating the race car on a flat, level surface. Make sure the wheels are pointed straight ahead. If you have toe-in set on the car, the toe needs to be returned to zero for now. It is also beneficial to have turn plates underneath the front wheels to reduce turning resistance. Remove the hub cover and install the caster/camber gauge onto the hub or spindle. In most cases, this is done by screwing the end of the gauge directly onto the threaded end of the hub/spindle. If your gauge uses a magnetic adapter, make sure the hub is clean and free of burrs, dings, and even excess grease so that you can be confident the gauge will be properly aligned.

The photography supporting this article was taken at the race shops of Mangum Motorsports, which competes in NASCAR's Late Model Stock Division. Mangum Motorsports uses a Rebco gauge with bubble levels. We are using the reference points on this gauge. Other gauges from different manufacturers may have different markings, but they all work on the same principle.

After the gauge is installed, begin by turning the wheel a preset amount. The exact amount isn't critical. Driver Shaun Mangum turns the wheel three quarters of a full turn since that is the most a racer will typically turn the steering wheel on the racetrack. If you are using turn plates, some gauge manufacturers recommend 20 degrees. It really doesn't matter as long as you are consistent and turn the tires the same amount every time you check the caster. Whether you turn the wheels to the right or left to begin the process also isn't important as long as you are consistent.

Check the Steering Axis Inclination (SAI) bubble at the bottom of the gauge. Turn the gauge until the bubble is centered over the "10." This is essentially your zero point. It's easier to communicate which way the bubble is moving rather than deal with positive and negative numbers. Now turn the centering screw on the gauge until the center of the caster bubble is over the zero mark.

Next, turn the wheel the same amount in the opposite direction. For example, Mangum begins by turning the steering wheel three quarters of a turn to the right. At this point he returns the steering wheel to the center and then turns it three quarters of a turn to the left. Readjust the gauge using the SAI bubble until it is back over the "10" and read the caster bubble. This is your caster angle.

Camber is defined as the angle of the tires relative to level ground. If the top of the tire is leaned in toward the center of the car, that is negative camber. If the top of the tire is leaned away from the car, that is positive camber. Camber helps maximize the tire's contact patch as it rolls through a turn. It's best to have a little negative camber on the right front and some positive camber on the left front. Just how much you want depends on your overall setup and the banking on the racetrack (higher banking requires less camber).

Checking your camber settings is even easier than checking caster. With the gauge on the wheel hub or spindle, rotate the gauge until the SAI bubble is centered over the "10" on the scale. With camber the wheel doesn't need to be moved. Simply read where the camber bubble is on the scale for your camber angle. There are two bubble levels for camber: one for positive camber and another for negative. It is easy to tell which you have because one bubble should always be off the scale.

Another thing you should remember to check is the change in caster and camber as the suspension moves up and down. Whenever you are checking bumpsteer with a bumpsteer gauge, take a few extra minutes to run a caster/camber loop at different levels of suspension compression and even droop. You don't need to go beyond what your suspension will normally see on the racetrack, but it will help you understand what's going on when the suspension is at different attitudes other than static ride height.